Atrial fibrillation (AF) is a common cardiac arrhythmia associated with an increased risk of stroke, heart failure, dementia, and mortality. Current treatments for AF, including antiarrhythmics, oral anticoagulants, and catheter ablation, have limited efficacy and carry significant risks, highlighting the need for prevention. Unfortunately, we currently lack effective interventions to prevent the onset of this common condition. Intervening on the cardiac substrate, characterized by atrial fibrosis and remodeling, to avert or reverse its development offers an attractive approach to AF prevention since this substrate is a key predisposing factor for AF onset. In our prior work in observational studies we have shown that increased body weight is a major risk factor for the development of AF. More recently, we also demonstrated in a randomized trial that individuals assigned to a Mediterranean diet enriched in extra-virgin olive oil had lower risk of AF than those assigned to a control diet. The pathophysiological mechanisms underlying the described associations are not completely understood, but changes in the atrial substrate are likely responsible. In this application, we propose to conduct an ancillary study to the recently-initiated PREDIMED-PLUS trial with the primary aim of assessing the effect of a weight-loss lifestyle intervention on the cardiac substrate of AF. The PREDIMED-PLUS study is a multi-center, randomized trial for the primary prevention of cardiovascular disease in 6,874 overweight and obese men and women with the metabolic syndrome in Spain. The trial will evaluate the effect of an intensive weight loss-focused lifestyle intervention program?including an energy-restricted Mediterranean diet, increased physical activity, and cognitive behavioral weight management?compared to a control intervention of low-intensity dietary advice on the Mediterranean diet. In the present application, we propose to conduct repeated assessments of the AF substrate performing repeated echocardiography and measurements of circulating biomarkers of pathways involved in the pathogenesis of the AF substrate (cardiac fibrosis, inflammation, oxidative stress, myocardial injury, atrial stretch) in 573 participants of the PREDIMED-PLUS trial with completed baseline echocardiographic exams. We will determine the effect of the intensive lifestyle intervention on cardiac structure and function, with a primary focus on the atria, and in biomarkers of AF-related pathways. Additionally, we will evaluate the association between the different components of the intervention (weight loss, compliance with the energy-restricted Mediterranean diet, increased physical activity) and changes in the atrial substrate. The proposed studies will determine the efficacy of an intensive lifestyle intervention on the pathophysiological substrate of AF and will inform the development of specific recommendations for the prevention of processes ending in the clinical onset of AF.